Chromium base alloys having excellent corrosion resistance and workability

ABSTRACT

CR-NI ALLOYS HAVING EXCELLENT WORKABILITY AND CORROSION RESISTANCE, WHICH COMPRISES 40-70% CHROMIUM, 2-7% MANGANESE, AND NICKEL AS THE REMAINDER. SAID ALLOYS MAY CONTAIN 0.5% MAX. CARBON OR 0.2% MAX. NITROGEN.

July 11, 1972 SATORU Mn'o E AL 3,676,110

CHROMIUM BASE ALLOYS HAVING EXCELLENT CORROSION RESISTANCE AND WORKABILITY 2 Sheets-Sheet 1 Filed. Dec. 8, 1970 .0 N E L D1 M A S X C o O O W SAMPLE No.4

|200C x SH-OQ 0 6 0 M H 30 E X 0 I. c 3 DI M 0 SR July 11, 1972 SATORU MITO ETAL 3,676,110

CHROMIUM BASE ALLOYS HAVING EXCELLENT CORROSION RESISTANCE AND WORKABILITY 2 Sheets-Sheet 2 Filed Dee.

00 W98 \l A k H T G N E RY TT U E mm m8 m T X T E fl lo 000 O m 9 8 1obo 1100 TEMPERATURE (C United States Patent US. Cl. 75-134 3 Claims ABSTRACT OF THE DISCLOSURE Cr-Ni alloys having excellent workability and corrosion resistance, which comprises 4070% chromium, 2-7 manganese, and nickel as the remainder. Said alloys may contain 0.5% max. carbon or 0.2% max. nitrogen.

BACKGROUND OF THE INVENTION The present invention relates to chromium-nickel alloys and more particularly to Cr-Ni alloys prepared by adding manganese and, if required, carbon or nitrogen to an alloy mainly consisting of chromium and nickel so as to elevate corrosion resistance and workability.

In the field of thermal engines and chemical apparatus, there is great demand for such alloys as are fully resistant to severe operating conditions, for example, continued use at elevated temperature particularly in a strongly corrosive atmosphere caused by chemical reaction to which they are unavoidably subjected for attainment of high heat efiiciency. To illustrate such severe conditions, there occurs in various types of burning furnaces using low grade heavy oil rapid corrosion of the walls and flues by vanadium pentoxide (V 0 used as catalyst. Also in an MHD generator, the furance walls are quickly corroded by, for example, sulfides of alkali metals contained in the seed. Further in automobiles, a thermal reactor intended to detoxicate gasoline engine exhaust by combustion is operated at elevated temperature, so that rapid corrosion results from lead oxide (PbO), a combustion product of tetraethyl lead contained in gasoline. While Cr-Ni alloys, for example, 50 Cr-SO Ni alloy in particular, can Withstand the aforementioned corrosion, they have the drawbacks that they present diificulties in forging and though casting may be available to Work them, the resulting product has little ductility and is likely to be broken due to brittleness. In general, chromium base alloys indeed present higher resistance to oxidation and corrosion with increasing chromium content, but become brittle to render forging difficult, possibly resulting in breakage due to said brittleness.

SUMMARY OF THE INVENTION The object of the present invention is to provide Cr-Ni alloys free from the above-mentioned shortcomings, prominently resistant to corrosion, readily forged and, what is better, sufficiently ductile to be saved from breakage due to brittleness. Namely, the object of the invention is to provide Cr-Ni alloys having excellent workability and corrosion resistance which consist of 40-70% chromium, 27% manganese and nickel as the remainder. For improvement of workability and corrision resistance, there may be added to said alloys, if necessary, 0.5 wt. percent max. carbon or 0.2 wt. percent max. nitrogen alone or in combination.

3,676,110 Patented July 11, 1972 "ice BRIEF EXPLANATION OF THE DRAWINGS FIG. 1 is a 300-fold magnified microscopic photograph of the crystal structure of Cr-Ni alloys prepared by the prior art process;

FIGS. 2 and 3 are 300-fold magnified microscopic photographs of the crystal structure of Cr-Ni alloys prepared by the present invention;

FIG. 4 is a photographic presentation comparing the deformation of prior art Cr-Ni alloys caused by hammering with that of the present invention; and

FIG. 5 is a curve diagram comparing the tensile strength and ductility at elevated temperatures of Cr-Ni alloys according to the present invention in comparison with that of the prior art.

DETAILED DESCRIPTION OF THE INVENTION The inventors have found that it is possible prominently to improve by addition of manganese the workability and extensibility of very oxidation-resistant high Cr-Ni alloys (containing considerable proportions of chromium) without degrading said oxidation resistance. Though prominently improved in resistance to corrosion by, for example, V 0 and PbO, the prior art Cr-Ni alloys which do not contain manganese present a double phase of double phase structure has been considered to obstruct the ductility of such alloys. In contrast, the Cr-Ni alloys of the present invention have the 'y-phase region broadened due to addition of manganese, with the resulting increase in ductility, extensibility and workability. It has also been discovered that incorporation of carbon or nitrogen alone or in combination in addition to manganese, more elevates workability than when manganese alone is used.

There will now be described the reason why limitation is imposed on the composition of Cr-Ni alloys. Discussion is first made on the necessity of limiting the content of chromium to the range of 40 to 70 percent by weight. Chromium, as well as nickel, constitutes the matrix of Cr-Ni alloys. Upon heating, chromium forms a stable thin layer of chromium oxide (Cr O on the surface of the alloy, to render it resistant to oxidation and corrosion. Since increased amounts of chromium prominently elevate this effect, it should be added at the rate of 40 percent by weight minimum. However, addition in excess of 70 percent by weight is undesirable due to degraded workability.

There will now be described the reason why addition of manganese is limited to 2 to 7 percent by weight. Manganese has been found to help chromium base alloys to have greater resistance to corrosion by, for example, V 0 and PbO and particularly good workability. While the proportion of chromium is less than 40 percent by weight, the crystal structure of Cr-Ni alloys exhibits a v-phase, so that addition of manganese is not significant. However, if the content of chromium rises higher, the alloy assumes an phase. Accordingly, there should be added at least 2 percent by weight of manganese so as to broaden the v-phase for easy forging. However, a large proportion of manganese leads to decreased oxidation resistance, depending on interrelationship with the chromium content. Therefore it is preferred that the upper limit to addition of manganese be set at 7 percent by weight. A particularly preferable range of Mn addition is between 4 and 6 percent by weight.

Like manganese, carbon spreads the 'y-phase to increase the ductility and workability of a Cr-Ni alloy in which it is incorporated. However, addition of carbon in excess of 0.5 percent by weight results in prominent crystallization of carbides to cause an undesirable decline in corrosion resistance. Nitrogen also broadens the 'y-phase to improve workability. However, excess inclusion of nitrogen leads to intergranular separation or cracks at the time of workup and in consequence reduced toughness. Accordingly, it is advisable to add nitrogen in 0.2 percent by weight at maximum. Where both C and N are used, the total content should preferably be 0.5 percent by weight or less.

Nickel, like chromium, constitutes the matrix of the subject alloy, and helps its structure to assume a 'y-phase. Therefore, its too small content causes the alloy to present an ot+'y phase, resulting in degraded workability. n the other hand, its too large proportion decreases the chromium content by that extent to reduce resistance to oxidation and corrosion. Further, the amounts of impurities, for example, -Fe and Co unavoidably entrained with the alloy should preferably be less than 1 percent by weight.

There will now be given the examples using the present invention. There were prepared rods of cast alloy 40 mm. in diameter and 200 mm. long by placing in a magnesium oxide crucible the base metals of chromium and nickel and other additions of manganese, carbon and nitrogen blended in the range of composition shown in Table 1 below, evacuating the crucible, introducing argon or nitrogen thereinto and melting the mass in said atmosphere. These cast alloy rods were subjected to solution treatment 3 hours at a temperature of 12000 C. for use as samples. These samples were tested at a temperature of 1200" C. on crystal structure, oxidation resistance, hot forging characteristics and tensile strength.

TABLE 1.ALLOY COMPOSITION (WEIGHT PERCENT) Component Cr Mn 0 N Ni Atmosphere Sample No 1 1 38.5 alance..- Vacuum.

4. 48.3 2. Argon gas 10 53. 9 5. 2 Nitrogen gas.

12 50.3 4.9 Vacuum 13 2 36. 8 2. 9 Argon gas.

16 70.6 9.0 do D0.

17 2 50.7 4.5 0.42 0.27 do Nitrogen gas.

1 (Jr-Ni alloys of the prior art. 1 Controls showing the compositions of other chromium-base alloys than those of the present invention.

FIGS. 1, 2 and 3 indicate the crystal structure at 1200 C. of the alloy samples Nos. 4 and 6 of Table 1. These figures show that while an 52 Cr alloy (FIG. 1) without Mn presented the prominent presence of an ot-phase (black dots scattered over the matrix), 48.3 Cr alloy (FIG. 2) containing 2.3 wt. percent and 51.7 Cr alloy (FIG. 3) containing 5.1 wt. percent Mn had the 'y-phase considerably broadened (white region) and that increased addition of Mn noticeably enlarged the 'y-phase. Said increased 'y-phase is considered to elevate the ductility, extensibility and workability of the alloy. This is also supported by the results of the later described tests on Workability and high temperature tensile strength.

Table 2 shows the resistance of the samples of Table 1 to oxidation and corrosion. The sample numbers correspond to those of Table 1.

TABLE 2.THE OXIDATION AND CORROSION RESIST- ANCES OF ALLOY Reduced volume due to V20 cor- PbO cor- Oxidation rosion at rosion at at 1,200 O.X 1,100 C.X 1,000 C. Test 30 hrs. 30 hrs. 30 hrs.

See footnotes end of Table 1.

Referring to Table 2, the oxidation test was conducted by cutting all the alloy samples into pieces 5 mm. x 10 mm. x 20 mm, exposing the pieces 30 hours to an atmosphere at 1200 C. and after cooling determining weight change by a chemical balance. Said weight change is presented on unit area basis.

In a corrosion test there were used alloy samples of the same dimensions as used in the oxidation test. For a V 0 corrosion test, the sample was coated on one side with a mixture of V 0 and 20 mol percent Na SO and heated 30 hours at a temperature of 1100 0., followed by determination of its weight change after cooling. Said weight change is indicated on unit area basis. With respect to a PbO corrosion test, the sample was coated, as in the V 0 corrosion test, on one side with PbO, heated 30 hours at 1100 C. and had its surface washed with an aqueous solution of 30% acetic acid. The weight change of the sample is also given on unit area basis. Table 2 shows that a Cr-Ni alloy containing Mn did not appreciably decrease in oxidation resistance but presented substantially the same degree of said resistance as that without Mn, whereas, with respect to corrosion by V 0 and P130, the Mn bearing alloy exhibited a several times stronger resistance than the latter alloy, and that a Cr-Ni alloy containing C or N indicated the same result.

Table 3 represents the results of a hammering workability teSt on the alloy samples of Table l as well as of determination on their resistance to deformation by Dynapak and maximum strain.

TABLE 3.THE WORKABILITY OF ALLOY See footnotes end of Table 1.

Referring to Table 3, the workability test was conducted by heating the alloy samples to 1200 C., forging them by hammering and qualitatively deciding workability from the condition of cracks appearing therein and the extent of deformation with respect to deformation resistance and maximum strain, determination was made by subjecting the sample to simple compression applied at a pressure of 4 kg. m./cm. in the axial direction using the Dynapak While maintaining the sample at 1200 C. 'FIG. 4 shows the deformation of the samples Nos. 2, 4 and 6 when they were forged under the same conditions. Table 3 above and FIG. 4 prove that addition of Mn prominently improved the workability of a Cr-Ni alloy even when the chromium content was high, that is, a decrease in deformation resistance and an increase in maximum strain, and that further addition C or N alone or in combination broadened the 'y-phase of such alloy, more elevating Workability.

There will now be described tests conducted at various temperatures on the high temperature tensile strength of Cr-Ni alloys using a high temperature tensility tester. Determination was made on the tensile strength, ductility and extensibility of the samples Nos. 2, 4, 6 and 7, the results being presented in FIG. 5. The curve numbers of FIG. correspond to the sample numbers. FIG. 5 shows that presence of Mn slightly reduced high temperature tensile strength, but increased ductility and extensibility at 1200 C. 3 to 8 times more than when it was not used.

This is evidence that addition of Mn renders a Cr-Ni alloy flexible, that is, more readily workable.

Accordingly, the Cr-Ni alloy containing Mn according to the present invention is Well adapted not only for forgings but also for castings.

What we claim is:

1. Cr-Ni alloys having excellent corrosion resistance and workability consisting on a weight basis of to chromium, 2 to 7% manganese, 0 to 0.2% nitrogen, 0 to 0.5% carbon and the remainder nickel.

2. Cr-Ni alloys as claimed in claim 1 wherein the amount of manganese is between 4 and 6 3. Cr-Ni alloys as claimed in claim 1 wherein the combined amount of nitrogen and carbon does not exceed 0.5

References Cited UNITED STATES PATENTS 2,23 8,160 4/ 1941 Doom 171 2,813,788 11/1957 Skinner 75-171 2,292,694 8/1942 Jerabek 75176 2,133,291 10/1938 Gordon 75-170 2,809,139 10/1957 Bloom et a1 75-171 X CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

